"Hercules is an open source software implementation of the mainframe
System/370 and ESA/390 architectures, in addition to the new 64-bit
z/Architecture", according to its
home
page. Hercules is quite popular and widely used.

A correspondent wrote me that,
">Flex ES is a proprietary implementation of
'Open 370' [?] ... intended to run on UnixWare and other Unix
platforms ... [and] on Linux as well. It gives about 20 MIPS of
S/390 ... per GHz of Intel silicon, is MP capable, and can run
any historical flavor of OS/390 (MVS-MVT-MFT), VM, or VSE
(DOS/VS-DOS-TOS) and can further emulate any historical flavor
of IBM DASD using a SCSI RAID containers and any historical tape
device using UNIX flat files. (It can attach real S/3x0
tapes and printers using a PCI channel emulator.) I have heard
that it can run configurations that IBM has not been able to
reconstruct for decades, and the savings on maintenance, space,
power and HVAC are absolutely immense." The best information
I've found on this so far is through
Open Mainframe.